Wednesday may be Global Running Day - but for those in the northeast U.S. and Canada, it may be one of the worst days of the year to lace up their sneakers and head outdoors. The air quality — deteriorated by wildfire smoke for much of the region — has become "unhealthy" according to national and global standards, leaving many to wonder how the grim haze will impact their exercise plans.

Is it safe to exercise outdoors when air quality is bad?
A 2021 study published in the European Heart Journal found that while physical activity is generally an important part of reducing the risk of cardiovascular diseases, in highly polluted air, it can do the opposite. The study, which looked at young adults exposed to particulate matter, found that those exposed to "high levels of PM2.5 or PM10, were at "an increased risk" of cardiovascular diseases. The study did note that higher-intensity exercises contributed to how much risk was posed.

Many local officials have warned their residents to avoid going outside if they can given current conditions. In New York, which is seeing some of the worst pollution conditions in the world because of the smoke brought in by the dozens of wildfires raging in Canada, Mayor Eric Adams said Wednesday the city is experiencing an "unprecedented event."

All five boroughs are under an air quality health advisory.

"We recommend vulnerable New Yorkers stay inside. And all New Yorkers should limit outdoor activity to the greatest extent possible," Adams said. "This is not the day to train for a marathon or to do an outside event with your children. Stay inside, closed windows and doors, and use air purifiers if you have them."

Even New York Road Runners, the nonprofit that puts on the annual TCS New York City Marathon, canceled its Global Running Day events on Wednesday and urged those in smoke-impacted areas to follow local health advisory guidelines.

IQAir, the company that tracks air quality data in cities globally, says that even moderate amounts of pollution - cities with air quality index scores between 26-50, can impact your ability to breathe outdoors. At "unhealthy" levels and above - 101 AQI and above - outdoor exercise is no longer encouraged.

Is it safe to exercise indoors when air quality is bad?
When AQI levels reach "unhealthy" levels, it's recommended that people exercise indoors. Typically, it's safe to exercise indoors during high pollution conditions, but that also depends on the venue in which you would be exercising. If windows and doors are constantly open, for example, there is a higher likelihood of the particulate matter getting indoors.

The Centers for Disease Control and Prevention suggests making the planned physical activity less intense and shorten the duration of the workout as well. Those who have asthma are urged to keep their inhalers on them at all times during times of poor air quality.

What is a safe air quality for running?
The best time to exercise outdoors is when your city's AQI is between 0 and 50, meaning the air quality is "satisfactory" and there is minimal risk of air pollution, experts say. Moderate AQI levels, from 51 to 100, are mostly acceptable, but the present pollution could pose a problem to those who have chronic health conditions or are more sensitive to pollution. The risk of health impacts only increases as AQI levels develop further.

How to stay safe when exercising in bad air quality
If you choose to go against warnings and engage in exercise outside, the American Lung Association suggests finding areas that may have lower pollution areas, such as nature trails, and avoid the busiest traffic times of the day, when cars add the most particulate matter to the air.

The organization also suggests opting for lower-intensity workouts. Higher-intensity workouts, such as long runs or sprinting, increases the amount you need to breathe, so keeping that activity to a minimum will "decrease the amount of unhealthy air that is inhaled," the ALA said.

"Deciding to take a long walk instead of a short run is another way that you can still enjoy the outdoors and minimize the possible lung damage."

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